Ideal LN Fault Code: Causes, Fixes & Repair Costs
What does the Ideal LN fault code mean?
The LN fault code on an Ideal boiler signals that the control board has detected a loss of burner flame — either the flame failed to establish properly during the ignition sequence, or it cut out unexpectedly while the boiler was running. As an 'L' code, this is a safety lockout: the boiler shuts itself down completely and will not restart until the underlying problem is resolved and the unit is manually reset. You may also see this displayed as FN or F2 on some Ideal models. The boiler behaves this way deliberately — allowing unburnt gas to accumulate would create a serious risk of explosion or carbon monoxide poisoning, so the system errs firmly on the side of caution.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Dirty or faulty flame-sensing electrode Common
The flame sensor electrode sits inside the combustion chamber and detects whether a flame is present by measuring a tiny electrical signal through the flame. Over time, a coating of soot, carbon deposits, or corrosion builds up on the electrode tip, making it unable to read the flame reliably. The PCB interprets this as 'no flame' and triggers a lockout — even when gas is actually burning. This is the single most frequent cause of a persistent LN code on Ideal Logic boilers.
- Low gas pressure or interrupted gas supply Common
If the gas pressure arriving at the burner drops below the required level (typically around 20 mbar), the flame either fails to ignite or extinguishes partway through the cycle. Common triggers include a pay-as-you-go meter running out of credit, a faulty or ageing gas meter, or a temporary interruption to the mains supply in your street. Checking whether other gas appliances in the property are working normally is a useful first indicator.
- Frozen or blocked condensate pipe Common
During cold UK winters, the external section of the condensate pipe — which carries acidic waste water away from the boiler — can freeze solid. This causes back-pressure that disrupts combustion and extinguishes the flame. Condensate traps can also become partially blocked with sludge if the boiler has not been serviced regularly. This cause is seasonal but extremely common and is one of the few issues a homeowner can safely address themselves.
- Blocked flue or restricted air intake Sometimes
The flue must be clear for combustion gases to escape and for fresh air to reach the burner. Bird nests, leaves, soot accumulation, or physical damage to the flue terminal can restrict airflow enough to destabilise or extinguish the flame mid-cycle. Properties where the flue terminal is close to a wall corner or recessed area are particularly susceptible.
- Faulty or failing fan Sometimes
The fan creates the correct air pressure for safe combustion and clears exhaust gases before ignition begins. A fan that is running too slowly, intermittently cutting out, or has seized entirely will produce incorrect flue pressure, which can prevent proper ignition or cause an established flame to go out. Strong wind gusting back down the flue can have a similar effect.
- Faulty gas valve Sometimes
The gas valve controls the precise flow of gas to the burner. If it is sticking, failing to open fully, or not modulating correctly due to wear or an internal fault, gas supply to the burner becomes inconsistent, leading to repeated flame loss events. This tends to appear on older installations or boilers that have not been regularly serviced.
- PCB (printed circuit board) fault Rare
The PCB is the boiler's central controller. In rare cases, a failing PCB misinterprets signals from the flame sensor or ignition system, generating a false lockout even when the burner and sensor are functioning correctly. PCB faults are relatively uncommon as a primary cause but become more likely after a power surge or on older boilers where capacitors and components have degraded.
How to fix it
- Check other gas appliances in your home DIY safe
Turn on a gas hob or check another gas appliance. If nothing is working, the issue is likely an interrupted mains gas supply — contact your gas supplier or check whether a prepayment meter has run out of credit. If other appliances are working normally, the fault is specific to the boiler.
- Check for a frozen condensate pipe DIY safe
Locate the condensate pipe — typically a white or grey plastic pipe (often 21.5 mm or 32 mm diameter) exiting the bottom or side of the boiler, usually running to an external drain or soil stack. If the weather has been at or below freezing, feel along the external section for a solid frozen section. Gently pour warm (not boiling) water over the frozen area until it thaws, then reset the boiler. Boiling water can crack the plastic pipe.
- Reset the boiler DIY safe
Once you have completed the checks above, press and hold the reset button (usually marked with a flame symbol) for 2–3 seconds. If the boiler fires up and runs normally, monitor it over the next hour. If the LN code returns quickly or the boiler fails to ignite at all, do not keep resetting — repeated resets without resolving the root cause can introduce more unburnt gas into the system. Limit resets to two or three attempts.
- Inspect the flue terminal for obvious external blockages DIY safe
From outside the property, visually check the flue outlet terminal for any obvious obstruction such as a bird nest, leaves, or physical damage. Do not attempt to dismantle the flue or reach into it. If you can see a clear blockage at the very end of the terminal, it may be possible to carefully remove it, but if the obstruction is internal or the flue shows damage, this must be dealt with by an engineer.
- Have a Gas Safe engineer inspect and clean or replace the flame sensor electrode Gas Safe engineer
A registered engineer will remove the electrode from the combustion chamber, clean off any carbon or oxidation from the sensing tip, check the electrode gap, and test the micro-amp signal it produces. If cleaning does not restore reliable detection, a replacement electrode is a straightforward and relatively affordable repair.
- Have a Gas Safe engineer test the gas inlet pressure Gas Safe engineer
Using a manometer, the engineer will measure the gas pressure at the boiler's inlet. It should typically be around 20 mbar under working conditions. If pressure is low, the engineer will determine whether the issue lies with the meter, the internal supply pipework, or the gas valve itself, and advise accordingly.
- Have a Gas Safe engineer inspect and test the fan Gas Safe engineer
The engineer will test fan speed, check the pressure differential across the fan (air pressure switch readings), and inspect for wear on the fan motor bearings. A slow or intermittently failing fan will be replaced. This involves working inside the sealed combustion section of the boiler.
- Have a Gas Safe engineer assess the gas valve and PCB if other causes have been ruled out Gas Safe engineer
If the electrode, gas pressure, fan, flue, and condensate pipe have all been checked and found to be in order, the engineer will test the gas valve for correct operation and modulation. As a last resort, the PCB will be assessed — a PCB replacement is the most expensive repair associated with this fault code and should only be considered once all other causes have been eliminated.
Parts you may need
- Flame sensor / ionisation electrode · from £35
- Ignition electrode · from £30
- Condensate trap / syphon · from £25
- Flue gas fan (boiler-specific) · from £120
- Gas valve (boiler-specific) · from £95
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting an Ideal boiler showing LN?
You can try resetting two or three times after completing the basic checks described above (gas supply, condensate pipe, flue). However, if the LN code returns each time, stop resetting and call a Gas Safe engineer. Repeatedly forcing the boiler to attempt ignition without resolving the cause means the ignition system is repeatedly trying to light a burner that cannot sustain a flame — at best this accelerates wear; at worst it points to a gas or combustion issue that needs professional diagnosis. If you ever smell gas, do not reset the boiler at all: evacuate the property and call the National Gas Emergency Service immediately on 0800 111 999.
What is the difference between LN and FN (or F2) on an Ideal boiler?
On Ideal boilers, codes beginning with 'L' are lockout faults — the boiler has shut down completely and requires a manual reset before it will attempt to restart. Codes beginning with 'F' are non-lockout faults that may clear themselves once the triggering condition goes away. LN and FN both relate to flame loss, but LN means the boiler has gone into full safety lockout, whereas FN may indicate a transient flame instability event. If your display shows FN repeatedly without locking out, it still warrants investigation, as it often precedes a full LN lockout.
How much does it typically cost to fix an Ideal boiler LN fault?
Most LN fault repairs fall in the region of £120–£350 for standard weekday call-outs. A flame sensor clean is at the lower end, while fan replacement or gas valve work typically sits in the middle of that range. A PCB replacement is the exception — this can cost £400–£500 including parts and labour and is relatively rare as the root cause. Emergency or out-of-hours call-outs will attract a premium on top of parts costs.
Can a frozen condensate pipe really cause an LN lockout on an Ideal boiler?
Yes, and it is one of the most common causes during cold snaps in the UK. When the condensate pipe freezes, acidic waste water backs up inside the boiler. This creates back-pressure in the heat exchanger and flue, which disrupts the combustion process enough to extinguish the burner flame. The boiler then detects flame loss and locks out with the LN code. Thawing the pipe with warm water and resetting the boiler usually resolves this immediately. To prevent recurrence, an engineer can lag the external pipe or reroute it internally.